To satisfy increased demand for bandwidth on current and future lightwave systems, new modulation formats are being developed that provide higher bit rates and greater spectral efficiency. Exemplary new modulation formats include differential phase shift keying (DPSK), differential quadrature phase shift keying (DQPSK), and the like. Referring to FIG. 1, on currently deployed systems, such as an optical system 10, these new modulation formats will be deployed with older modulation formats. For example, the optical system 10 includes terminals 12, 14 with intermediate optical add-drop multiplexers (OADMs) 16, 18, 20, 22. In the optical system 10, there are existing 10G channels 24 with 40G channels 26, 28, 30 of two different modulation formats, i.e. 40G channel 28 is DPSK, and 40G channel 30 is duo-binary (DB). Accordingly, in the future 40G channels with other modulation formats will be added to the optical system 10 as well as 100G channels and the like. Also, as shown in the optical system 10, the combination of channel types (bit rate and modulation format) will change between optical add-drop multiplexers (OADM) 16, 18, 20, 22.
In general, different modulation formats have different sensitivities to optical signal-to-noise ratio (OSNR) and different tolerances to fiber non-linear impairments. Also, their tolerance to non-linear fiber impairments differ among different types of optical transmission fiber, e.g. Non-Dispersion Shifted Fiber (NSDF), Truewave, Large Effective Area Fiber (LEAF), Lambda Shifted (LS), and the like. Because of these differences, optimization of performance (i.e., reach) of each modulation format requires that channels of different formats be launched at different channel powers. The optimum launch channel power depends on the channel type (bit rate and modulation format) and fiber type. Therefore, as channels propagate through the optical system 10, the power of a channel of a given bit rate and modulation format should be changed when it reaches a fiber of a different type. Also, to achieve optimal performance for all channels, the amount of change could differ from one channel type to another.
Currently, if several different types of channels are propagating together in the same system and they see a change in fiber type then all of their powers are changed by the same amount, in dB, to account for the differences in tolerance to non-linear fiber impairments in the second fiber type. This is done even though channels of different bit rates and modulation formats might, optimally, be changed by different amounts in the second fiber to optimize performance. If the power of a channel is less than optimum at some places between its source and destination then the performance and/or reach of the channel is reduced because the OSNR at the receiver is reduced. Conversely, if the power of a channel is higher than optimum at some places between its source and destination then the performance and/or reach of the channel is reduced because of higher penalties that are due to fiber non-linear impairments.
Conventionally, there are no algorithms which automatically set the power of a channel according to the channel type and fiber type.